Unsaturated acids, such as methacrylic and acrylic acids, acrylonitrile, and the esters of such acids, such as methyl methacrylate, are widely used for the production of corresponding polymers, resins and the like. Various processes and catalysts have been proposed for the conversion of alkanoic acids, such as acetic acid or propionic acid, and formaldehyde to the corresponding unsaturated monocarboxylic acids, e.g., methacrylic acid, by an aldol-type reaction. Generally, the reaction of a carboxylic acid and formaldehyde takes place in the vapor or gas phase while in the presence of a basic or acidic catalyst.
The literature is replete with disclosures of the reaction of aliphatic carboxylic acid compounds with formaldehyde to produce alpha, beta-ethylenically unsaturated aliphatic monocarboxylic acid compounds of one more carbon atom than in the saturated carboxylic acid. For every molecule of alpha, beta-ethylenically unsaturated aliphatic monocarboxylic acid produced there is one molecule of water by-product. It is necessary to separate the alpha, beta-ethylenically unsaturated carboxylic acid compound, formaldehyde and the starting unsaturated carboxylic acid.
In the case of methacrylic acid, this means that the methacrylic acid must be separated from propionic acid, formaldehyde and water. This separation presents several problems since each of the components are water soluble and because propionic acid and methacrylic acid have boiling points that are so close that it is difficult to fractionate one from the other. Further, the separation is complicated by the fact that methacrylic acid has a tendency to homopolymerize, and formaldehyde, if water is removed from the system, also has a tendency to homopolymerize.
Of the various a1pha. beta-ethylenically unsaturated compounds, it is generally recognized that methacrylic acid has one of the greatest tendencies to polymerize and it is extremely difficult to handle at elevated temperatures. In this regard, we have found that the presence of certain reaction by-products greatly increase the propensity of methacrylic acid to homopolymerize. Specifically, alpha-, beta- unsaturated ketones, i.e., ethylisopropenyl ketone and 2,5-dimethylcyclopenten-1-one, have been shown to greatly increase the degree of methacrylic acid homopolymerization. Additionally, methacrylic acid, propionic acid and formaldehyde individually form binary azeotropes with water. The boiling points of the three binary azeotropes are within 1.degree. F. of each other and are thus exceedingly difficult to separate.
The following table lists boiling points and weight percentages of binary azeotropes of water and methacrylic acid, propionic acid and formaldehyde at 760 mm Hg.
______________________________________ Wt % Wt % H.sub.2 O B.P. .degree.F. ______________________________________ Methacrylic acid 23.1 76.9 210.7 Propionic acid 17.8 82.2 210.4 Formaldehyde 18.25-21.0 79.0-81.75 210.4 ______________________________________
In somewhat greater detail, the invention relates to a process for an aldol-type condensation of a saturated aliphatic monocarboxylic acid compound and an aldehyde wherein said monocarboxylic acid is propionic acid and said aldehyde is formaldehyde. As is well-known, an aldol-type condensation can be base-catalyzed and is subject to ready dehydration if the .beta.-hydroxyl group is adjacent to an .alpha.-hydrogen atom. The product is an .alpha., .beta.-unsaturated acid of one more carbon atom than the original unsaturated aliphatic monocarboxylic acid, when the reacting aldehyde is formaldehyde. The reaction using propionic acid and formaldehyde is: EQU CH.sub.3 CH.sub.2 COOH+HCHO .fwdarw.CH=C(CH.sub.3)COOH+H.sub.2 O
While the prior art has indicated that it is possible to carry out these reactions utilizing various catalysts, none of these references disclose specifically the use of a silica catalyst comprising at least one cation of a Group I or Group II metal and a silica support wherein the cation is present in a concentration of 0.001 to 0.2 equivalents per 100 grams silica support on a dry solids basis in the presence of a C.sub.6 to C.sub.12 hydrocarbon.
A careful review of the prior art has failed to disclose any examples wherein addition of a C.sub.6 to C.sub.12 hydrocarbon to the aldol-type condensation reaction improved the product yield.
In the prior art a number of methods which use solvent materials have been taught to separate the unreacted propionic acid and unreacted formaldehyde from the aqueous effluent resulting from vapor phase condensation of propionic acid and formaldehyde. U.S. Pat. No. 3,414,485 teaches use of a selective organic solvent to recover methacrylic acid from an aqueous reaction product effluent. Suitable organic solvents include o-, m- and p-xylene, toluene, n-octane, mono-chlorobenzene, methylamylketone, ligroin and methyl methacrylate monomer. U.S. Pat. No. 3,478,093 teaches use of a lactam having 4 to 7 ring members and a hydrocarbon radical substituent on the nitrogen atom as an extraction solvent to separate methacrylic acid from aqueous mixtures. U.S. Pat. No. 3,781,332 teaches use of a dual mixture containing methyl or ethyl methacrylate and not more than 50% of xylene, ethyl benzene or a mixture thereof. U.S. Pat. No. 4,040,913 teaches a decantation method wherein an organic solvent extracts methacrylic acid and azeotropes with propionic acid. The aqueous raffinate is separated by decantation. U.S. Pat. No. 4,142,058 teaches use of a mixed solution of methyl methacrylate and toluene to separate methacrylic acid from an aqueous solution containing acetic acid. U.S. Pat. No. 4,147,721 teaches use of methyl n-propyl ketone as a solvent to recover methacrylic acid from an aqueous reaction product.
However, introduction of a C.sub.6 to C.sub.12 hydrocarbon into the reactor to increase reaction yield has not been previously taught. Preferably the C.sub.6 to C.sub.12 hydrocarbon azeotropes with propionic acid to permit separation of propionic acid from methacrylic acid by downstream distillation.
The general object of this invention is to provide an improved method of reacting a saturated monocarboxylic acid compound, with formaldehyde to obtain an increased yield of an alpha, beta-ethylenically unsaturated aliphatic monocarboxylic acid compound of one more carbon atom than the starting saturated monocarboxylic acid compound. A more specific object of this invention is to provide an improved method of preparing methacrylic acid from propionic acid and formaldehyde which results in an increased yield of the desired methacrylic acid.
The general object of this invention can be attained by injecting a C.sub.6 to C.sub.12 hydrocarbon into the reaction of a saturated aliphatic monocarboxylic acid compound, and a formaldehyde compound in the presence of a catalyst comprising a silica support and at least one cation of a Group I or Group II metal in a concentration of about 0.001 to 0.2 cation equivalents per 100 grams by weight silica support on a dry solids basis at a temperature of from about 280.degree. C. to about 500.degree. C. under vapor phase conditions. Yields have increase by about 10% (e.g., 30% to 33%). Suitable C.sub.6 to C.sub.12 hydrocarbons are substantially non-reactive, water-immiscible compounds capable of breaking a water azeotrope of saturated aliphatic carboxylic acid compound. In a preferred method of operation, upon distillation, a major proportion of the ethylenically unsaturated monocarboxylic acid compound remains in the bottom of the column, and a major portion of the water, a portion of the formaldehyde compound and a major portion of the C.sub.6 to C.sub.12 hydrocarbon are removed overhead. In separation of the reaction products of propionic acid and formaldehyde in the production of methacrylic acid and water using a silica catalyst, we have found it advantageous to recycle the recovered C.sub.6 -C.sub.12 hydrocarbon through the reactor. We have also found that by removing a side stream below the top of the distillation column, it is possible to recycle a substantial portion of unreacted formaldehyde and propionic acid together with the hydrocarbon to the reactor and avoid the polymerization and plugging of the distillation column by polymerized formaldehyde as is pointed out in application Ser. No. 624,049, Pat. No. 4,599,144, filed on even date in the names of Baleiko, et al, incorporated herein by reference. In a more preferred method of operation, the unreacted propionic acid, formaldehyde and C.sub.6 to C.sub.12 hydrocarbon are recycled to the inlet ports of the reactor and employed to produce methacrylic acid.